Nico Rosberg - Is he a true #1 driver? (poll)

The Williams this year seems to be the fourth best car so far (behind the Brawn, Red Bull and Toyota) and isn't far off the pace. But the results haven't come. Rosberg has shown speed but just hasn't been able to deliver the results. Is it the car or the driver? Nakajima is having a terrible season, so Nico is clearly getting more out of it than his teammate. But is he WDC material if they can manage to gain the couple of tenths they would need to fight for wins?